Monday, June 17, 2013

Shear Keys

We're supposed to be in the final home stretch for opening the new Bay Bridge, and this weekend the New York Times carried this story: In California, Bolts May Hold Up a Bridge in More Ways Than One

The new bridge, whose foundation will reach the bedrock underneath, will be the world’s largest self-anchored suspension bridge, an unusual design that contributed to its high cost. Equipped with the latest antiseismic technology, the bridge was designed to last 150 years by remaining intact in a major earthquake.

That is why the failure of some high-strength steel bolts attaching shock-absorbing devices called shear keys to a concrete crossbeam under the roadway raised alarms. When workers tightened the 17- to 24-foot-long bolts in March, 32 in a batch of 96 snapped. Engineers blamed hydrogen-assisted cracking, in which atoms of hydrogen infiltrate steel and make it brittle, but they have yet to determine its cause.

Don't miss the nifty infographic: Solving a Structural Issue, which explains the concept of the Shear Keys with a nice diagram.

Shear keys and bearings allow limited and controlled movement at the joints of different sections of the bridge during seismic events.

Of course, since the whole point of the new bridge is to address seismic issues, it's natural that engineers are intently focused on the seismic aspects of the new design.

I've long been fascinated with this part of civil engineering, since I live in such a seismically active area. Ten years ago, I was privileged to have a window seat for the construction of Oakland's Cathedral of Christ the Light, a beautiful building, and once again a structure which was constructed to replace a structure that collapsed in the same great Loma Prieta earthquake.

Among the interesting aspects of the Cathedral of Christ the Light is the use of enormous wooden beams for the main space, since the natural flexibility of the wooden beams is supposed to make a more earthquake-friendly structure. Interesting, since as the NY Times article about the bridge observes,

the eastern half, a cantilever design that runs from the island to Oakland, was deemed beyond retrofitting. Its main problem was that it was anchored in treated Douglas fir trees that were used as pilings in clay and mud.

Poor trees: sometimes they are the blessing; sometimes they are the curse.

A more common earthquake technique, used in both the bridge and the cathedral, is the use of special seismic bearings. In the cathedral, as the manufacturer points out,

The new cathedral is located just 4.7 km from the Hayward fault and uses Friction PendulumTM seismic isolation bearings to protect it from damage during a 1000 year earthquake. The bearings support a 120 foot tall main sanctuary, which incorporates glulaminated timber, reinforced concrete, high strength steel, and aluminum and glass to create a light-filled structure.

I remember watching the bearings being installed; they were quite fasinating to observe.

And, of course, don't find the language confusing: "a 1000 year earthquake" is not an earthquake which lasts for 1000 years (though in fact the faults in California are nearly constantly moving, and it's isn't an inaccuracy to describe such continual movements in such a way), but rather an earthquake of such severity that we might expect it would occur once every 1000 years.

Meanwhile, back over the water, the engineers are pointing out that nothing is ever perfectly safe, and rather than ask whether the new bridge is flawless, we should be asking whether it is (a) safe enough, and (b) safer than the current bridge: Bay Bridge: Open new span with or without bolt repairs, experts say

If a moderate temblor were to strike after traffic moves onto the new bridge but before the retrofit is done or any other bolts are replaced, the new bridge is still at least twice as safe as the 77-year-old span motorists use today, Seible and Fisher said.

And some engineers just love the fact that the Bay Area is building new bridges: My Word: Stop all the negativity about beautiful bridge project

We in the Bay Area are renowned for our creativity, our leadership in technology and our eagerness to move forward. This bridge exemplifies all of these strengths. And let's not forget that Caltrans originally proposed a conservative, although arguably bland, simple viaduct structure to replace the existing bridge.

We rejected that and are much better off for it.

Although there are no guarantees in seismic engineering because temblors are inherently random, computer modeling and analysis has become so sophisticated that we have a truly remarkable understanding of the predicted performance of large structures in seismic events.

There's no doubt that the attention on seismic design has brought amazing improvements in construction techniques. But I'm also pleased to see that there's always room for improvement, and people are already actively studying the results of these recent large public projects, trying to learn from them and figure out how to do still better in the future.

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